Superconductivity is that characteristic of certain materials which permits them to conduct electric currents without resistance. A superconducting material exhibits this characteristic only when its temperature is below the superconducting critical temperature of the material and then only if it is not subject either to a magnetic field greater than the superconducting critical magnetic field of the material or to an electric current greater than the superconducting critical current of the material. Accordingly, a superconductive material can be quenched (i.e. returned to a resistive state) by increasing the temperature, magnetic field, or current to which the material is subjected above the critical temperature, critical magnetic field, or critical current. In a given material, quenching of the superconductivity may occur abruptly or more gradually depending upon the breadth of its superconducting transition state with respect to temperature, magnetic field or current.
Briefly stated, with regard to metallic superconductors it is known that selected parent-metals, either pure or preferably containing minor solute-metal alloying additions, are capable of being alloyed with other reactive-metals and forming superconducting compounds or alloys that have a high current-carrying capacity. The parent-metals niobium, tantalum, technetium, and vanadium can be reacted or alloyed with reactive-metals, such as tin, aluminum, silicon, and gallium to form superconducting alloys, such as the intermetallic Nb.sub.3 Sn.
Additionally, it is known that the superconductive characteristics of various parent-metals can be improved by first alloying the parent-metal, i.e., niobium, tantalum, technetium, and vanadium, with a minor amount of a solute-metal having an atom diameter of at least 0.29 angstroms larger than the diameter of the parent-metal atom prior to reacting the reactive-metal with the parent-metal. A broad disclosure of various parent-metals, solute-metals, and reactive-metals can be found in U.S. Pat. No. 3,416,917, which is incorporated herein by reference. Of the various possible combinations of the materials mentioned above, it is known that niobium is an extremely useful parent-metal, particularly when reacted with the reactive-metal, tin to form Nb.sub.3 Sn. This intermetallic compound has superior superconducting properties. More specifically, U.S. Pat. No. 3,429,032, which is incorporated herein by reference, discloses improved critical currents in superconductive Nb.sub.3 Sn alloys formed when the parent-metal niobium also contains zirconium, in an amount of at least 0.1 weight percent and up to an amount equivalent to the ratio represented by the formula Nb.sub.2 Zr, and is heated in the presence of excess tin, and a non-metal selected from the group consisting of oxygen, nitrogen, and carbon.
Metallic superconductors, particularly those comprising Nb.sub.3 Sn, have been fabricated in various forms, particularly wires and tapes, in efforts to produce devices such as high field superconducting electromagnets. Superconductive metallic devices of laminated construction having an elongated tape or strip configuration and the methods of producing such superconductive tapes are well known. For example, U.S. Pat. No. 3,661,639, incorporated by reference herein, discloses improved superconductive tapes, and methods of forming the improved tapes. U.S. Pat. No. 3,537,827, incorporated by reference herein, discloses improvements in laminating superconductive tapes and methods for producing the laminated tapes. One method for obtaining superconducting tape in a continuous fashion is that wherein a tape of a preselected parent-metal, such as niobium or niobium alloy, (e.g. an Nb--Zr--O alloy) is continuously led through a bath of molten reactive-metal such as tin or a tin alloy, such as a tin-copper alloy. The niobium tape picks up a thin coating of the reactive-metal from the molten bath and the tape is subsequently heated in a reaction furnace to cause formation of the superconductive Nb.sub.3 Sn alloy on the surface of the parent-metal tape. It is also known that the reactive-metals can be alloyed to further improve the superconductive tape. For example, the critical current density of Nb.sub.3 Sn has been improved by making copper additions of up to 45 weight percent copper in the reactive-metal tin for coating on niobium tape as disclosed in, "Enhancement of the Critical Current Density in Niobium-Tin" J. S. Caslaw, Cryogenics, February 1971, pp. 57-59.
Methods for making superconductive joints between superconductive tapes, such as Nb.sub.3 Sn tapes, are well-known as described in U.S. Pat. Nos. 5,109,593, 5,134,040 and 5,239,156. Most of the development of joining techniques related to superconductive tapes appears to have been directed toward joining techniques for forming continuous lengths of tape. However, it is also desirable to form larger superconductive articles from these tapes. For example, larger superconductive articles made from these superconductive tapes could be used for electromagnetic shielding for sensitive electromagnetic devices that must be isolated from large magnetic fields. Therefore, it is desirable to fashion superconductive tapes into larger superconductive articles.